Struggling boxing manager Moe is having lunch with several associates who are angry at him for their low payment and threatening to quit on him. When he notices their shy waiter (Curly) goes into a violent fugue whenever he hears the song "Pop Goes the Weasel" which is the case when he knocks out three associates and later his boss. Moe also takes notice of a fiddler (Larry) who happens to be playing the potent tune at the restaurant. Seeing dollar signs in the uncontrollable waiter, Moe quickly recruits the two unsuspecting cohorts and preps them for the boxing world. Curly's boxing handle becomes "K.O. Stradivarius", and with Larry in tow--playing "Pop Goes the Weasel" at every boxing match, Curly becomes the number-one contender for the heavyweight championship.
Before the matches occur, while Curly is doing the rowing exercise under the supervision of Moe, on a rural road, they notice a young lady, whose car is stuck in the ditch. Moe wants to help her out, at first not letting Curly get involved, stating that "BOXING AND WOMEN DON'T MIX", However, Moe asks him to help with the problem, and requests Larry to play the "Weasel" tune. When Larry does this again for the second time, Curly does succeed in getting the car out of the ditch, and hops in the car with the lady driving away from the rural area. Curly is found on the day of the big match with the lady in the dressing room, causing Moe to hit him, telling him not to interfere with women and to go upstairs into the arena.
All goes well until the night of the highly anticipated World Championship match with Killer Kilduff (Al Hill). Only a few moments into the first round, Kilduff plants a left hook at Curly, sending him into the crowd, landing on Larry and crushing his violin. Frantic, Larry scurries the streets, looking for anything that is playing "Pop Goes the Weasel," while Curly is being battered by the boxer. Larry finds a radio playing the tune, takes it and heads back to the arena. Larry arrives at the arena a few minutes later with the radio. The song ends moments later just as Curly is about to knock out Kilduff, when the radio announcer is about to tell the children's story involving "Peter Rabbit", causing Moe to crash the radio on Larry's head, ruining the box. Moe sends Larry back out to find something else playing "Pop Goes the Weasel".
Larry manages to come across a politician's campaign truck blaring the tune from its speakers and "race-drives" it to the arena, crashing through a side wall. Curly is just about ready to throw in the towel until he hears "Pop Goes the Weasel". The wobbily boxer comes to his feet with renewed energy, and knocks out Kilduff in a matter of seconds, winning the fight. The song continues, however, and as they celebrate in the ring, Curly knocks out Larry and Moe and begins to advance on the camera as the short ends.